The invention relates to an electric circuit breaker with means for adjusting the contacts, comprising for each electric phase moving levers having main contacts and arc-breaking contacts co-operating with main contacts and arc-breaking contacts provided on a terminal bar.
It is known from the prior art that industrial low-voltage circuit breakers accomplish two tasks in the main: allowing the flow of a rated current and interrupting higher overload or short-circuit currents. These two tasks, investigated from the point of view of the electric contacts, are mutually conflicting.
Thus, in order to guarantee low consumption and a low working temperature during the flow of the rated current, the electric contacts require a small resistivity. On the other hand, for the purpose of interrupting overcurrents, the requirement demanded of the electric contacts is a high resistance to the electric arc generated during the interruption of an overcurrent or short-circuit and hence high resistivity.
Being impossible to satisfy with the same material the requirement of low resistivity and high thermal resistance simultaneously the tasks of transmitting the rated current and of interrupting overcurrents are accomplished by two distinct contacts made from different materials.
The function of carrying the rated current is accomplished by the first contacts or main contacts, made from softer material. The function of interrupting the overload or short-circuit current is accomplished by second contacts or arc-breaking contacts, made from tougher materials.
Metal inserts, constituting the main contacts and moving arc-breaking contacts, are inserted at various heights on moving levers supported by cranks pivoting on the box of the circuit breaker. Corresponding metal inserts, constituting the main contacts and the fixed arc-breaking contacts, are inserted on the end of upper terminal bars of the circuit breaker.
A known linkage generates, during the opening phase, the movement of the contact-carrying levers which effects the switchover from the main contacts to the arc-breaking contacts. To obtain correct operation of the circuit breaker it is necessary for the switchover from the main to the arc-breaking contacts to take place in accordance with an accurate time sequence. The desired accuracy in the time sequence is obtained by adjustment of the distance of the corresponding fixed and moving contacts.
At the present time, this adjustment is performed by rotating the moving contact-carrying levers, by virtue of a known adjustment device, acting on each contact-carrying lever.
The operation of adjusting the position of the individual contacts of the complete circuit breaker is made extremely arduous and lengthy due to the presence inside the circuit breakers of three or four electric phases, each made with several contact-carrying levers, for example eight levers, each having a main contact and an arc-breaking contact.
These operations are usually carried out by specialist staff using for example suitable feeler gauges inserted between the corresponding fixed and moving arc-breaking contacts, when the circuit breaker is closed and therefore when the main contacts are shut. The adjustment operation is performed by rotating the individual contact-carrying lever until the feeler gauge is clamped between the arc-breaking contacts with the desired force.
As may easily be appreciated, this operation requires a certain accuracy in its performance and hence, given also the considerable number of contacts present in the circuit breaker, makes this phase of the manufacture of the circuit breaker extremely lengthy.